Summary of Work: Magnetic fields associated with the use and transmission of electricity have been reported to depress nocturnal levels of melatonin in rodents, especially hamsters. However, recent research sponsored under the NIEHS EMF RAPID program suggests that the melatonin effect in rodents is less robust than originally reported. In vitro models have shown that high melatonin levels can slow the growth of human breast cancer cells thus providing some support for the association between children living in homes close to power lines and increased cancer rates. A 25 week study was completed in which rats were exposed to light at night lowering melatonin levels while control rats were exposed to normal light. Cancer of the mammary gland was induced by methyl nitrosourea and the incidence of cancer was compared in control rats versus those with lower nocturnal melatonin levels. The study with histopathological examination and data analysis has been completed. The manuscript has been submitted for publication. Since breast cancer continues to increase in women, is one of the leading causes of death and since the factor(s) leading to the increasing rates is not known, this project is of great public health significance.